Calibration of resistance factors for drilled shafts for the new FHWA design method : [tech summary].
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2013-01-01
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Abstract:The load and resistance factor design (LRFD) has been increasingly used and has become mandatory for the design of all bridge projects funded by the Federal
Highway Administration (FHWA). Compared to the allowable stress design (ASD) method, LRFD can achieve a compatible reliability between the bridge
superstructure and substructure. The uncertainties of load and resistance are quantifi ed separately and reasonably incorporated into the design process.
Therefore, this reliability-based design approach will generally produce a more effi cient and consistent design than the traditional ASD factor of safety approach.
To achieve these goals, many researchers have been working to develop a reasonable way to implement the LRFD method in bridge substructure design and to
determine appropriate resistance factors for diff erent regional soil conditions. Although the American Association of State Highway and Transportation Officials
(AASHTO) LRFD specifi cations were approved for use in 1994, the implementation of these specifi cations for bridge design has been slow. The resistance
factors (ɸ) proposed in the specifi cations were derived from ASD safety factors to maintain a consistent level of reliability with past practices. As a result, little
improvement has been made toward a more efficient design. One outstanding problem with the resistance factor calibration is the lack of a good database. In
the latest edition of the AASHTO specifi cations, a signifi cant number of resistance factors in the foundation design were still selected based on the calibration.
Several research efforts have been carried out to calibrate the resistance factors for drilled shafts from case histories available nationally. Currently, AASHTO
specifi cations recommend using total resistance factors (ɸ) for single drilled shafts in an axial compression range from
0.40 to 0.60 at a reliability index (β) of 3.0, depending on different soil conditions. These factors were calibrated based
on drilled shaft databases that were collected from various sites that do not necessarily reflect the local soil condition
of individual states. As a result, the resistance factors recommended by the AASHTO LRFD design code should be
verified and recalibrated to account for local soil conditions and design experience.
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